As a conventional process for producing a phosphate hexafluoride, for example, a process for producing LiPF6 may be a process of dissolving lithium chloride in hydrogen fluoride and then adding, thereto, phosphorus pentachloride (Non-Patent Document 1 described below). In this process, phosphorus pentachloride is added directly to hydrogen fluoride. Since phosphorus pentachloride has strong hygroscopic property, phosphorus pentachloride contains water contained originally therein and water absorbed from air at the time of the addition. LiPF6 contains the waters, thereby being hydrolyzed. Thus, a lithium oxyfluoride such as LiPOF4 is generated.
In order to solve such a problem, for example, Patent Document 1 described below discloses a method of cooling a gas generated by reaction between phosphorus pentachloride and hydrogen fluoride into the range from −40 to −84° C. to remove phosphorus oxyfluoride, and then introducing the gas into hydrogen fluoride in which lithium fluoride is dissolved.
Patent Document 2 described below discloses a process of allowing a phosphorus compound and hydrofluoric acid to react with each other to produce phosphorus pentafluoride, and introducing the phosphorus pentafluoride into a solution of a fluorine compound in anhydrous hydrofluoric acid, thereby producing a phosphate hexafluoride compound. Furthermore, Patent Document 2 states that as the phosphorus compound, PCl5, PBr5, P2S5, or the like is used without using any oxide or oxychloride of phosphorus, which gives water as a byproduct.
As described above, in conventional processes for producing LiPF6, in the course of the production, water is involved; therefore, there is caused a problem that a hydrolyzate produced on the basis of the water results in the decrease of purity.
[Patent Document 1] JP-A-60-251109
[Patent Document 2] Japanese Patent No. 2987713
[Non-Patent Document 1] Fluorine Chemistry Vol. 1